An EL device comprising a light emitting layer formed of an inorganic compound and interleaved between upper and lower insulator thin films is excellent in luminance characteristics and stability upon driven on AC current. EL devices fabricated through a fabrication process where all process steps are carried out with thin-film technologies are now used for a variety of displays. One basic arrangement of such a light emitting device is shown in FIG. 2.
This light emitting device has on a glass substrate 21 a multilayered film structure comprising a transparent electrode 22 formed of ITO or the like, a thin-film first insulator layer 23 and a thin-film light emitting layer 24 composed of an electroluminescence-producing fluorescent material such as ZnS:Mn, and further comprising on the light emitting layer 24 a thin-film second insulator layer 25 and a back electrode 26 formed of an Al thin film or the like, and makes use of light emitted out of the transparent glass substrate side.
Each of the thin-film first and second insulator layers is a transparent dielectric thin film made up of Y2O3, Ta2O5, Al2O3, Si3N4, BaTiO3, SrTiO3, etc., and formed by a sputtering or evaporation process.
These insulator layers perform important functions in limiting currents passing through the light emitting layer to contribute to improvements in the stability of operation and light emission of the thin-film EL device, and protecting the light emitting layer against moisture and harmful ion contamination to improve the reliability of the thin-film EL device.
However, such a device has some practical problems. One problem is that it is difficult to reduce the dielectric breakdown of the device to nil over a wide area, resulting in low yields, and another is that the applied driving voltage necessary for the device to emit light becomes high because voltage is dividedly applied to the insulator layers.
To solve the dielectric breakdown problem, it is preferable to use an insulator material having good dielectric strength properties. To provide a solution to the light emission-driving voltage problem, it is preferable to increase the capacity of the insulator layers, thereby reducing the proportion of the voltage dividedly applied to the insulator layers. In view of the principles of operation of such a thin-film EL device of the AC driving type, the current passing through the light emitting layer contributing to light emission is virtually proportional to the capacity of the insulator layers. To decrease the driving voltage and enhance the luminance of light emission, it is therefore of vital importance to increase the capacity of the insulator layers.
For this reason, it is attempted to use a ferroelectric PbTiO3 film of high dielectric constant formed by a sputtering process as an insulator layer, thereby achieving low-voltage driving. This PbTiO3 sputtered film shows a dielectric strength of 0.5 MV/cm at a relative permittivity of 190 at most. However, the temperature of the substrate must be elevated to about 600° C. for PbTiO3 film formation, and so it is difficult to apply the PbTiO3 film to the fabrication of hitherto thin-film EL devices using a glass substrate. Besides, a SrTiO3 film formed by a sputtering process, too, is known in the art. This SrTiO3 sputtered film has a relative permittivity of 140 and a dielectric breakdown voltage of 1.5 to 2 MV/cm. This film is formed at 400° C. However, the practical use of the film for a thin-film EL device using a glass substrate offers a problem because an ITO transparent electrode is reduced and blackened during film formation by sputtering.
One possible approach to solving this problem is to use for the glass substrate a glass material that has a high softening point and can be treated at high temperature. In this case, however, the substrate costs much, and the upper limit to the treatment temperature is again 600° C. as well.
Another approach is to make insulator layers thinner. However, the ITO film is susceptible to dielectric breakdown at its edge because of the insufficient dielectric strength of such thinner insulator layers. This is an obstacle to development of large-area and large-capacity displays.
Thus, a conventional thin-film EL device must be driven at high voltage, resulting in the need of using a costly driving circuit of high dielectric strength. This unavoidably makes displays costly and large-area displays hardly achievable.
Among EL devices known to solve these problems, there is an EL device wherein a thin-film light emitting layer 34, a thin-film second insulator layer 35 and a transparent second electrode 36 are stacked on a multilayered ceramic structure comprising a ceramic substrate 31, a thick-film first electrode 32 and a first insulator layer 33 of high dielectric constant, as shown in FIG. 3.
In this EL device, a low-temperature sintering Pb perovskite based material is used for the first insulator layer. However, this material must be used with an increased thickness because of its insufficient dielectric strength. For this reason, it is impossible to reduce the emission start voltage down to a sufficiently low level.